As a method for controlling torque of such a synchronous motor at a high speed and with a high accuracy, there is known a vector control method in which a current of a synchronous motor is controlled based on a rotating coordinate system synchronized with a position of a rotor detected by a position sensor. The position sensor has problems such as that a mounting space and cost are increased and that a use environment of the motor is limited by a use environment of the position sensor.
As one measure to solve these problems, a position-sensorless vector control method is in practical use, in which rotor position information is estimated based on an induced voltage of the motor calculated from a current and a voltage of the motor and a motor constant such as a winding wire resistance.
Further, because the induced voltage is proportional to a rotation speed of the motor, when the motor speed is low, for example, at a time of start-up, the induced voltage is excessively low, thus an estimation accuracy of the rotor position is low. Therefore, there is known a method in which a synchronous operation is performed at the time of starting the motor and in which the operation is shifted to position-sensorless vector control when the rotation speed increases to an appropriate speed. Specifically, in this method, a synchronous operation mode is used to perform a synchronous operation at the time of start-up of the motor with a predetermined current being supplied to the motor winding, and a frequency of the current is gradually increased so that the motor is accordingly accelerated. Then, after the motor is accelerated to a predetermined rotation speed at which the position of the rotor can be estimated, the mode is switched to a position-sensorless vector control mode.
However, in this method using the two modes, a change occurs in a phase of the current flowing through a motor winding when the mode is switched to the position-sensorless vector control mode, and the change in the phase causes vibration, noise, and a rapid acceleration of the motor.
Therefore, as a technology to address these issues, a method of PTL 1 is conventionally proposed, for example. In PTL 1, an axial error is first estimated without using a sensor during a synchronous operation, and feedback control is performed such that a phase difference between the estimated axial error and a current instruction value becomes identical. Next, after the estimated axial error is reduced to within a predetermined range, the operation is switched from the synchronous operation to sensorless control. After that, the phase is gradually changed to a current phase of position-sensorless vector control.
However, in the conventional method in PTL 1, because the phase is gradually changed to the current phase of the position-sensorless vector control after the operation is switched from the synchronous operation to the position-sensorless vector control, it takes time to shift to the perfect position-sensorless vector control.